1. Field of the Invention
The present invention relates to scanning type display optical systems in which a two-dimensional image is formed by deflecting and scanning a light beam in two dimensions.
2. Description of Related Art
Until now, a number of retinal scanning display optical systems rendering images directly on an observer's retina have been proposed, that utilize the effect of after-images and deflect and scan a light beam emitted from a light source in two dimensions with an optical deflection device.
For example, U.S. Pat. Nos. 5,557,444, 5,467,104, and 5,903,397 disclose the principle of a retinal scanning display. Such displays form images on a primary image-forming plane with a light beam that is deflected in two dimensions, and form a two-dimensional image on the observer's retina with an eyepiece optical system
Japanese Patent Application Laid-Open No. 2001-4955 discloses a display optical system rendering images directly on an observer's retina, that includes at least one prism. The prism has at least three surfaces, that is, an incident surface, an internal reflective surface and an emergent surface. Deflected light beam is reflected at least three times inside the prism, and at least one of the reflective surfaces has a certain optical power.
Moreover, a display optical system of the two-dimensional scanning type has also been proposed, which forms two-dimensional images by scanning the image plane two-dimensionally with a spot. Here, with a retinal scanning display optical system rendering images directly on an observer's retina, the moving characteristics of the scanning spot and the image plane size on the image plane change due to defocusing of the scanning spot on the image plane that occurs when a light beam is deflected. This leads to changes in the field angle and to image distortions.
In order to address this problem, Japanese Patent Application Laid-Open No. H10(1998)-68876 discloses an optical system that is provided with such telecentricity that the moving characteristics of the scanning spot on the image plane do not deviate from fθ characteristics as a result of defocusing when deflecting a light beam.
Furthermore, Japanese Patent Application Laid-Open No. H8(1996)-146320 discloses a two-dimensional scanning optical system, in which a light beam emitted from a light source is deflected two-dimensionally and scanned two-dimensionally over a scanned plane by a scanning lens having f·sin θ characteristics as distortion characteristics. This is supposed to correct distortions of the displayed image with the f·sin θ characteristics of the scanning lens and an electrical correction.
However, U.S. Pat. Nos. 5,557,444, 5,467,104, and 5,903,397 do not disclose a specific optical structure of the retinal scanning display.
Also, in structures using reflective surfaces inside the prism as disclosed in Japanese Patent Application Laid-Open No. 2001-4955, optical loss becomes a problem, because the reflectance of the optical member is ordinarily low compared to the transmittance. In particular when three or more reflective surfaces are used, the optical loss becomes large.
Furthermore, even though the two-dimensional scanning optical system proposed in Japanese Patent Application Laid-Open No. H10(1998)-68876 is telecentric and changes in the moving characteristics of the scanning spot on the image plane due to defocusing are small, its application is for light sources of infrared wavelengths of at least 10 μm, and it is not suited for use in the visible spectrum. Moreover, this publication does not disclose anything regarding the use of a plurality of light sources of different wavelengths.
Moreover, since in the two-dimensional scanning optical system proposed in Japanese Patent Application Laid-Open No. H8(1996)-146320, the size of the image plane changes and the characteristics on the image plane change when the image plane comes out of focus, it is difficult to use it for applications in which constant distortion characteristics are desired.
Moreover, it is known that image distortions occur on the image plane when a two-dimensional image is formed by scanning and deflecting a light beam in two dimensions. Such image distortions include trapezoidal distortion, constant-speed scanning distortion, rectilinear scanning distortion, and furthermore TV distortion, in which the frame of the image formed on the image plane is distorted. In particular if the direction from which the light beam coming from a light source is incident on the light deflection member is oblique with respect to the deflection axes of the light deflection member, the TV distortion and the trapezoidal distortion become large.
However, with the electrical correction employed in the two-dimensional optical system proposed in Japanese Patent Application Laid-Open No. H8(1996)-146320, an effective correction of the TV distortion is difficult. And Japanese Patent Application Laid-Open No. H10(1998)-68876 does not disclose anything regarding the correction of TV distortion.